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Old 18th April 2004, 09:27 PM   #11
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I would definitely go for Eva's proposal.
This solution should work quite reliable and with
comparative low heat....
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Old 18th April 2004, 09:31 PM   #12
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Adding windings in opposite direction as proposed by janneman should also be fine, if you have torroids.
If you do not have torroids you may probably not have enough spare winding area...
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Old 19th April 2004, 12:53 PM   #13
Dazzzla is offline Dazzzla  Australia
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Adding windings in opposite direction as proposed by janneman should also be fine, if you have torroids.
The transformers are iron core so winding extra turns may be to hard.

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Take first transformer and wire both secondaries in series, then wire them in series with the primary to get a 300V to 230V autotransformer. Use this autotransformer to reduce mains from 230V to 176V. Use these 176V to power the second transformer and you will get about +-38V after rectification
I seem to recall reading somewhere that auotransformers for mains voltage are illegal in Australia, better not use this method just encase.
Please correct me if I'm wrong.

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If you are talking about a circuit "similar" to this one, remember that you will have to dissipate "15V x Max_current" in the 2955/3055 transistors. For 8 ohms load during continuous sinus signal at 50W, it gives 3.5 A, thus more than 50 W to dissipate for positive supply and the same for negative supply! So a big heat sink will be required.
I think I'll try this way using Tip2955 and Tip3055 which both have a total power dissipation of 90W.
Thanks for all your help.
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Old 19th April 2004, 01:39 PM   #14
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Why do you want to wind reverse?
why dont you wind extra primary? if you add severals turn to the primary the voltage at secondary will drop
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Old 19th April 2004, 02:24 PM   #15
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Yes, that was my idea, the extra winding should be in series with the primary, and in phase, not reversed. I goofed.

Jan Didden
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Old 19th April 2004, 02:33 PM   #16
Dazzzla is offline Dazzzla  Australia
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Why do you want to wind reverse?
By winding turns in reverve on the primary, you effectivly reduce the magnetic field. Its nearly the same as removing the the primary turns, turn for turn . In an ideal world you mite have say 400 turns on the primary and by winding 400 turns in the reverse direction you could null the secondary voltage (0vac), you would probably create quite alot of heat and destroy the transformer. Fire and death!
So I wouldn't advise doing this!

Mains voltage is DANGEROUS
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Old 19th April 2004, 04:45 PM   #17
Eva is offline Eva  Spain
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Powering consumer equipment directly from an autotransformer may be forbidden due to the absence of mains isolation, but in the solution I proposed the output is taken from the second transformer so it's perfectly isolated from mains. The purpose of the first transformer is solely to step down the mains voltage seen by the primary of the second transformer


EDIT:

Dazzla : Sorry to say you are wrong. Adding turns to the primary in reverse direction actually increases secondary voltages and also increases magnetic field at the risk of core saturation. In the other hand, adding turns to the primary in the same direction reduces secondary voltages and also reduces magnetic field so core saturation will never happen and core losses will be lower
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Old 19th April 2004, 07:51 PM   #18
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NO NO NO NO javascript:smilie('')
bigeyes If you wind extra turns for the primary you must NOT reverse.

Transformer is a ratio device, voltage is proportional to the number of turns.

If you add severals turn to a winding ( not reverse for substraction, but in the same direction ) you will get a winding with higher voltage.

so if you add severals turns consecutive to the secondary you will get a higher voltage at the output winding.

so if you add severals reverse turns to the secondary you will get a lower voltage at the output winding.

If you add severals consecutive turns to the primary the primary will have a higher nominal voltage but when connected to a standard voltage power line the output will drop

If you add severals reverse turns to the primary the primary will have a lower nominal voltage and when connected to a standard voltage power line the output will raise, and also the core will saturate and that may run into overcurrent and overheat
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Old 19th April 2004, 07:57 PM   #19
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...looks like one misunderstanding is hunting the other......
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Old 19th April 2004, 09:56 PM   #20
fab is offline fab  Canada
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Quote:
Originally posted by Dazzzla


I think I'll try this way using Tip2955 and Tip3055 which both have a total power dissipation of 90W.
Thanks for all your help.
Adding to what I indicated on that, remember that a typical 8 ohms speaker has a varying impedance with frequency over the audio band. It can be easily 4 ohms at specific frequencies. Of course, this high amount of current only appears during big transients. I would consider parallelling the 2955/3055 with another one to be on the safe side. It seems a lot of components but this fully regulated supply (including power stage) will most likely improve the sound performance (it is present in extremely expensive hi-fi power amps like Classé Omega, Halcro dm series).
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